Abundances of refractory ions in Beta Pictoris exocomets

Abstract

Beta Pic is a young, A5V star, known for harbouring a large number of exocomets, which frequently transit the star and produce absorption signatures. The physical and chemical properties of these exocomets can be probed by the recently introduced curve of growth approach, which enables column densities measurements in exocomets using observations in numerous spectral lines. Using this approach, we present a new study of archival spectra of Beta Pic obtained with the HST, the HARPS spectrograph, and at the Mont John University Observatory, aimed at constraining the abundance of refractory ions in Beta Pic exocomets. 29 individual objects are studied, all observed in FeII lines (used as a reference ion) and at least one other species (Ni II, Ca II, Cr II...). We find that the refractory composition of exocomets is overall stable, especially for singly ionised species, and consistent with solar abundances. This validates the use of the curve of growth approach to study exocometary composition. We also show that some ions, such as Ca II, are significantly depleted compared to solar abundances, allowing us to constrain the ionisation state in Beta Pic exocomets. We find that most refractory elements (Mg, Ni, Fe...) are split in similar fractions between their first and second ionisation states, with the exception of Ca, mostly ionized twice. A strong correlation between the Al III/Fe II ratio and radial velocity is also found, showing that the most redshifted exocomets tend to be more ionised. These results open the way for further modelling, in order to better understand the physical processes that influence the composition and shape of exocometary tails.